Dust emission and star formation of the Stephan`s Quintet compact group of galaxies

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Understanding the intergalactic medium (IGM) gas cooling processes, which are necessary to fuel star formation in galaxies, and the effect of galaxy--galaxy and galaxy--IGM interactions, which modify stellar and gas distributions of galaxies in groups and clusters, is vital to construct realistic models of galaxy formation and evolution. The Stephan's Quintet (SQ) compact group of galaxies is a natural laboratory for studying these phenomena because the galaxies of this group are heavily interacting between each other and with the group IGM. Furthermore its vicinity allows to study the details of the interaction phenomena, which are believed to be much more common in the early universe, and its compactness on the sky permits studies of diffuse components associated with the group IGM. In this thesis we present an analysis of a comprehensive set of MIR/FIR observations of Stephan's Quintet, taken with the Spitzer Space Observatory. The emission seen at these wavelengths is produced by dust particles and can be used to trace star formation events, AGN activity and also hot gas cooling, in the case dust emission is powered by collisions between plasma particles and dust. Applying a novel fitting technique to the Spitzer FIR maps, we have been able to separate the different sources of dust emission in this group and perform their photometry at FIR as well as MIR wavelengths. Our study has revealed for the first time the presence of a luminous and extended component of infrared dust emission, not connected with the mainbodies of the group galaxies, and roughly coincident with the X-ray halo of the group. We fitted the inferred dust emission spectral energy distribution of this extended source and the other main infrared emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering the dust and PAH emission, taking into account dust collisional heating and heating through UV and optical photons. Combining the fraction of dust luminosity powered by UV photons, as derived from the SED fitting, with the UV luminosity directly observed on the GALEX FUV map of SQ, we estimated the star formation rate (SFR) for each dust emitting source, thus providing a complete picture of star formation in SQ embracing obscured and unobscured components. The total SFR of SQ is $7.5~{rm M_odot/yr}$, similar to the value expected for non interacting galaxies of the same mass of SQ galaxies. However the star formation sites are found mainly at the periphery of the galaxies or in the intergalactic medium, at variance with the usual pattern of star formation in field galaxies which is typically distributed in the central regions or main bodies of galaxies. Despite the unusual location of star formation sites, we have found that, for the brightest sources in SQ, the SFR per unit physical area is similar to that characteristic of disk galaxy star formation regions when compared to the corresponding gas column density on a Kennicutt--Schmidt diagram. We also show that even though the detected extended component of dust emission trace the distributed group star formation, available sources of dust in the group halo can provide enough dust to produce up to $L_{IR}approx10^{42}~{rm erg/s}$ powered by collisional heating. This amount, several times higher than the X-ray halo luminosity, could provide an important cooling mechanism for the IGM hot gas. At the end of the thesis we present a theoretical model of a high velocity shock, similar to the one occuring in SQ IGM, taking into account dust cooling and dust destruction. This model shows that, although the efficiency of dust cooling drops quickly because of dust removal by sputtering, the gas cooling time is reduced by a factor of 2-3, compared to the case where only radiative cooling is considered.